Steam cracking is a well-known process and is described in U.S. Pat. No. 3,641,190 and British Pat. No. 1,077,918, the teachings of which are hereby incorporated by reference. In commercial practice, steam cracking is carried out by passing a hydrocarbon feed mixed with 20-90 mol % steam through metal pyrolysis tubes located in a fuel fired furnace to raise the feed to cracking temperatures, e.g., about 1400.degree. to 1700.degree. F. and to supply the endothermic heat of reaction.
The fluid coking process for the production of fuels, such as gas oil and naphtha is well known and is disclosed in U.S. Pat. No. 2,881,130, the teachings of which are hereby incorporated by reference. Integrated fluid coking and coke gasification processes are also known and disclosed, for example, in U.S. Pat. Nos. 3,661,543 and 3,816,084, the teachings of which are hereby incorporated by reference.
Fluid coking processes for the production of chemicals and chemical intermediates are also known, see, for example, U.S. Pat. Nos. 2,846,360 and 2,871,183. Generally, when it was desired to produce chemicals rather than fuel oils, heretofore, the fluid coking process usually included a high temperature transfer line cracking zone and a fluidized bed coking zone.
It is also known to introduce a small amount of hot solids into a gas-solids separation zone, such as the cyclone separator, used to separate entrained solids from the vaporous coker product, so as to prevent coke deposition on the walls of the cyclone separator, see for example, U.S. Pat. Nos. 2,763,601; 2,859,168 and 2,943,993. This treatment is known as scouring. For scouring coke the amount of heat to be added need only be enough to uphold the temperature of the vapors and prevent them from depositing coke whereas the heat load is much greater for steam cracking.
The present process relates to and is an improvement on U.S. Pat. No. 4,297,202. This application also relates to U.S. Ser. No. 272,503 filed on June 11, 1981.
In cocracking, a carbonaceous material is coked in a first stage fluid coking zone and the resulting vaporous coker conversion product is passed to a second stage reaction zone where it is heated in the presence of steam to a temperature adapted to crack the same to products including low molecular weight unsaturated hydrocarbons. This is achieved by injecting a stream of hot solids at a sufficiently high temperature and in sufficient amount into the vaporous coker conversion product passing to a gas-solids separation zone such as the cyclone separator. The hot solids may be supplied by a coke gasification zone.
In cocracking there are two stages in the reactor, which are operated at different temperatures and which accomplish different purposes, viz., coking and steam cracking. These two stages are carried out in the same environment and in close proximity to each other. The first stage is suitably carried out in a large fluidized bed of solids which is fed a preheated coking feed, typically a vacuum residuum. The second stage is suitably effected in a smaller, dilute phase which is fed vapor from the first stage. The main flow of solids which supplies heat for both stages, flows counter-current to the flow of hydrocarbon feed.
Although the temperatures for the two stages are disparate, it was found that these two reactions can be coordinated and caused to take place in the same vessel.
This problem of temperature control is more acute when high flow rates of heat-carrying solids to the dilute phase are used in order to reach the preferred, higher than minimal steam cracking temperatures, inasmuch as these solids--after heat exchange with the coking vapors and separation from product gas in the gas-solids separation zone--normally pass into the coking zone and could heat the latter excessively.
One way of maintaining suitable temperatures in the coking zone involves injecting water into the bed to lower and control bed temperature. In the following discussion it will be useful to consider this as the base case. However, this is not an efficient use of heat. Also, the reactor vapors from the bed have a tendency to cause coke build-up on the walls and equipment in the dilute phase because the vapors are at their dew point.
The subject invention provides a novel technique and apparatus to establish and control the temperatures of the two zones and alleviate both of these problems.